《机械专业英语》课件第14章.ppt

1、Unit 14TEXT LEARN TO SPEAK NEW WORDS AND EXPRESSIONS NOTES EXERCISES READING MATERIAL PROFESSIONAL WORDS AND PHRASES Tool LifeThe higher the cutting speed,the shorter the tool life.By tool life is meant the time of cutting until the tool becomes too worn to do its work effectively and must be sharpe

2、ned.Although it cannot be universally applied,many cutting operations on most materials obey the empirical relationship：VTn=CWhere V is the cutting speed in surface meters per second,T is the tool life in cutting seconds,and C is a constant,subject to the variables of work material,cutting tool mate

3、rial,tool shape,and other factors.The constant C is equal to the cutting velocity for a tool life of l min.Figure 14-1 shows a typical tool life curve that follows this mathematical relationship.Fig.14-1 Tool life curves for turning steel bar If such a machining formula is available,the production p

4、lanner can use it to work out the lowest cost of machining.Obviously the fastest possible machining speed will give the lowest cost per cubic millimeter of chips,if no other factors are involved.However,this ideal condition may mean frequent tool changes and sharpening.In the time required to change

5、 tool bits perhaps 25 cubic millimeter or more of chip making may be lost.The production planner must then select a compromise speed.He may select the speed that will give a tool life extending over a full shift of production or over the whole production run,if it is reasonably short.Or he may balan

6、ce the cost of tool change and tool sharpening against the speed of machining to provide a minimum overall cost.LEARN TO SPEAKMr.Zhang:Hello,Mr.Cheng.Nice to see you again!Mr.Cheng:Im glad to meet you.Mr.Zhang:What are you doing?Mr.Cheng:I am preparing an experiment of tool life.Mr.Zhang:Oh,tool lif

7、e?What does it mean?Mr.Cheng:Tool life is the time of cutting until the tool becomes too worn to do its work efficiently and must be sharpened.Mr.Zhang:How can we decide that the tool must be sharpened?Mr.Cheng:If the tool is broken or the cutting operation is very difficult and the machining qualit

8、y is not satisfactory,the tool must be sharpened.Mr.Zhang:It seems difficult to judge it.Mr.Cheng:Not the case.It is very easy for a skilled worker.Mr.Zhang:Oh,I see.I think I have a long way to go.Mr.Cheng:Do not worry.You can do your best if you work hard.Mr.Zhang:Youre very kind.Mr.Cheng:Thats OK

9、.NEW WORDS AND EXPRESSIONS obey v.服从，顺从 empirical adj.经验的，实验的 subject n.题目，主题，科目 adj.从属的，易受的 vt.使受到，使服从 typical adj.典型的，标准的 curve n.曲线 v.绘曲线 planner n.计划者 cubic adj.立方体的，立方的 involve vt.包含，涉及 ideal adj.理想的 compromise n.折衷 NOTES1By tool life is meant the time of cutting until the tool becomes too worn

10、 to do its work effectively and must be sharpened.刀具耐用度是指刀具磨损过大以致无法有效地切削而必须刃磨之前的切削时间。by+名词A+is meant+名词B “A是指B”或“A就是B”这是科技英语中对某一术语下定义时常用的句型。主语是名词B，放在谓语动词is meant 的后面，是一个倒装句。例如：By cutting speed is meant the rate at which the work passes the cutter.切削速度是指工件通过刀具的速率。too+形容词或副词+to v.“太(以致)不”这种句型中，后面的to v

11、.有否定的意义。如：This problem is too difficult to understand.这个问题太难，理解不了。Modern high-strength alloys are often too hard to machine.现代高强度合金常常硬得难以机械加工。2Although it cannot be universally applied,many cutting operations on most materials obey the empirical relationship：虽然下述经验公式不能普遍适用，但对大多数材料的许多加工是成立的：代词it是指后面主

12、句中的the empirical relationship。英语中有时人称代词先出现，而所代的名词在后，译成汉语时，要作适当处理。3Where V is the cutting speed in surface meters per second,T is the tool life in cutting seconds,and C is a constant,subject to the variables of work material,cutting tool material,tool shape,and other factors.式中V是切削速度，以米/秒表示；T是刀具耐用度，以

13、切削秒数表示；C是一个常数，随工件材料、刀具材料、刀具形状和其它因素而变化。where引出的从句，说明上面的公式。这种场合where 一般译成“式中”、“其中”。本句中subject to是形容词短语，说明前面的名词a constant;subject to 表示“以为条件”，“受制于”。4Obviously the fastest possible machining speed will give the lowest cost per cubic millimeter of chips,if no other factors are involved.显然，若不涉及其它因素，以可能达到的

14、最高切削速度来加工，就能使切削每立方毫米切屑的成本最低。possible用在形容词的最高级后面，用来加重语气，可译成：“尽可能”、“能时”。如：the best means possible 尽可能好的方法 EXERCISES1Discuss the following questions in English:(1)How to define the metal cutting operation called“turning”?(2)How to define the term“cutting speed”?(3)How to define the word“machinability”(

15、切削性)?(4)Can you say the two classes of metallic materials that are used to make metal-cutting tools?What are they?2Use English for replacing Chinese or use Chinese for replacing English.(1)刀具的耐用度与很多因素有关。(2)加工时的切削速度对刀具的寿命有很大的影响。(3)制造刀具的材料有很多种，如高速钢、硬质合金、陶瓷等。(4)The lower the cutting speed,the longer th

16、e tool life is.(5)Tool life curve follows mathematical relationship.(6)The cutting velocity is very important for metal to be cut.3Place a“T”after sentences that are true and an“F”after those that are false.(1)Rake is the slope of the top of the tool away from the cutting edge.()(2)The chip movement

17、 and pressure causes the dome to become soft,so that particles of it break away,and become welded to the chip and to the body of the work-piece.()(3)Continuous chip formation is associated with brittle work-piece material,and cutting tools with a small rake angle.()(4)In the formation of the chip by

18、 tear,there is a general movement of the chip over the tool face.()(5)The purpose of the first,or roughing cut,is to remove the surface of the work-piece as rapidly as possible.()(6)The purpose of the last or finishing cut is to produce a work-piece of the required size that has the quality of surfa

19、ce finish desired.()4Fill the blanks with the words given below:being,inside,tear,outside,rake,mechanical,physical,clearance,having,shear.(1)The force F acting upon the tool can be reduced by_ a large rake angle,and the tendency of welding to occur can be eliminated by making the cutting tool from a

20、 material that is non-metallic.(2)In practice,the_ of the chip display the“step”produced by the intermittent slip,but the_ of the chip is burnished smooth by the action of the chip rubbing upon the tool face.(3)_ involves compression of the work-piece material adjacent to the tool face,so that a cra

21、ck runs ahead of the cutting tool,and towards the body of the work-piece.(4)The force F acting upon the tool is reduced by increasing the _ angle.(5)The rake angle controls the chip formation,and is in turn governed by the_ properties of the material being cut.READING MATERIALThe Shape of the Cuttin

22、g ToolIt is obvious that the narrower the blade of a cutting tool,the less force or power will be required to“wedge”it through the material to be cut.Therefore,when cutting soft materials with a cutting tool made of a much harder and stronger substance,the blade can be sharpened to a long thin edge.

23、As the hardness of the material to be cut increases,however,the strength of the cutting edge must also be increased.Fig.14-2 shows forces action on a lathe tool.Cutting a soft iron bar,for example,will cause a fine steel blade to dull immediately,even though the steel in the blade is harder than the

24、 metal being cut.If you were to examine the edge of this blade under a glass,you would see that it is either bent or broken.The shape of a cutting tool,then,helps to determine the strength of a cutting edge.This explains why a knife edge will dull almost upon contact with metal while the more obtuse

25、 cutting edge of a cold chisel will stand up to continuous pounding into metal.Fig.14-2 Forces action on a lathe tool The cutting-edge of any metal-cutting tool must be strong and well supported(see Fig.14-3).Remember that a tremendous force is exerted against a cutting-edge during the machining pro

26、cess.Fig.14-3 Acarbide tip fastened to a tool holder The action of the cutting tool on the metal being cut determines the kind of chip that will be formed.Chips show whether a machine tool is being operated correctly.They also show whether the cutting tool is correctly ground to the shape and sharpn

27、ess required for the job.PROFESSIONAL WORDS AND PHRASES blade n.刀刃，刀口，叶片 wedge n.楔，斜铁 vt.楔入，劈开 substance n.物质 dull v.变钝，弄钝 adj.钝的 immediate adj.直接的，立刻的 immediately adv.立即，马上，直接地 examine v.检查，调查，考试 explain v.解释，说明 obtuse adj.钝的，不快的 chisel n.凿，錾 tremendous adj.极大的，惊人的 sharpness n.锐利，尖锐 cold chisel 冷錾stand up to 经受(得住)，耐whetheror 或